Method for bonding wafers and structure of bonding part

ABSTRACT

A method for bonding wafers includes forming a first bonding part on a surface of a first wafer by stacking a diffusion preventing layer formed of a material having low wettability with AuSn above the first wafer and forming a bonding layer on a surface of the diffusion preventing layer such that the bonding layer stays back of an edge of the diffusion preventing layer, forming a second bonding part on a surface of a second wafer, and bonding the first bonding part and the second bonding part by eutectic bonding with an AuSn solder under a condition that the first wafer and the second wafer are opposed to each other.

BACKGROUND

1. Technical Field

The present invention relates to a method for bonding wafers and astructure of a bonding part, and more particularly, to a method forbonding two wafers by AuSn eutectic bonding and a structure of itsbonding part.

2. Related Art

When bonding parts of two wafers are bonded, AuSn eutectic bonding isused in some cases. The AuSn eutectic bonding means a bonding methodusing an eutectic reaction of a metal and AuSn and is used in fields ofadvanced MEMS packaging and three-dimensional stacking technique.According to the AuSn eutectic bonding, the AuSn is melted like asolder, and it has an advantage that bonding can be easily performedeven when a surface of the bonding part is rough like a plated layer.

Thus, when the bonding parts of the wafers are bonded by the AuSneutectic bonding, as for one wafer, an adhesive layer formed of Cr isprovided in some cases as a layer (lowermost layer) which is in contactwith the wafer (refer to patent document 1). This adhesive layerfunctions to enhance adhesiveness between the bonding part and thewafer.

However, when the molten AuSn solder comes in contact with the adhesivelayer and the AuSn diffuses into the adhesive layer when the bondingparts are bonded, adhesive force between the adhesive layer and thewafer deteriorates, and reliability reduces in the bonding part.

In addition, as for the other wafer, a penetration wiring whichpenetrates the wafer is connected in some cases to the bonding part(electrode) provided on a surface of the wafer (refer to patent document2). In this case, the bonding part is previously provided on the surfaceof the wafer, a via hole is formed in the wafer so as to penetrate it,the penetration wiring is formed in the via hole by a plating method,and the penetration wiring is bonded to the bonding part. However, thebonding part is formed of the same material (such as Al) as thepenetration wiring in the via hole in general, so that when the via holeis formed by etching the wafer, or when the penetration wiring is formedby plating, the bonding part could be etched and damaged by an etchingsolution or a plating solution (acid liquid in general).

[Patent Document 1]: Japanese Patent No. 3303227

[Patent Document 2]: Japanese Unexamined Patent Publication No.2007-311771

SUMMARY

According to one or more embodiments of the present invention, a methodfor bonding wafers and a structure of a bonding part is capable ofpreventing the bonding part from deteriorating due to AuSn. According toone or more embodiments of the present invention, a structure of abonding part is capable of preventing the bonding part from beingdamaged when a penetration wiring is formed in a via hole.

A method for bonding wafers according to one or more embodiments of thepresent invention includes steps of forming a first bonding part on asurface of a first wafer by stacking a diffusion preventing layer formedof a material having low wettability with AuSn above the first wafer andforming a bonding layer on a surface of the diffusion preventing layerin such a manner that the bonding layer stays back of an edge of thediffusion preventing layer, forming a second bonding part on a surfaceof a second wafer, and bonding the first bonding part and the secondbonding part by eutectic bonding with an AuSn solder under a conditionthat the first wafer and the second wafer are opposed to each other.

According to the method for bonding the wafers in one or moreembodiments of the present invention, the bonding layer is provided onthe surface of the diffusion preventing layer formed of the materialhaving low wettability with AuSn in such a manner that it stays back ofthe edge of the diffusion preventing layer, so that the molten AuSnsolder is not likely to spread on the surface of the diffusionpreventing layer when the first bonding part is bonded by the AuSneutectic bonding, and the AuSn solder is prevented from flowing towardthe first wafer. Therefore, even when a layer which is likely todeteriorate due to the diffusion of the AuSn is provided between thefirst wafer and the diffusion preventing layer, the layer is not likelyto deteriorate because the AuSn does not diffuse thereto. There is acase where the adhesive layer containing Cr as its main component isformed on the surface of the first wafer. This is because Cr is high inadhesiveness with the wafer, so that the first bonding part can befirmly adhered to the first wafer. In this case, when the AuSn diffusesinto the adhesive layer, the adhesiveness of the adhesive layerdeteriorates, but according to one or more embodiments of the presentinvention, the AuSn can be prevented from diffusing into the adhesivelayer, so that the adhesive layer can be prevented from peeling off. Inaddition, in a case where an actuator is formed under the first bondingpart, the AuSn solder can be prevented from going beyond the diffusionpreventing layer and attaching to the actuator.

According to the method for bonding the wafers in one or moreembodiments of the present invention, an AuSn solder layer is previouslyformed on at least one surface of the first bonding part and the secondbonding part by alternately stacking Au and Sn or with an AuSn alloy,and the first bonding part and the second bonding part are bonded by theAuSn eutectic bonding with the molten AuSn solder layer. Thus, since theAuSn solder layer is previously provided, it is not necessary to applythe AuSn solder to either bonding part when the first bonding part andthe second bonding part are bonded.

According to one or more embodiments of the present invention, thediffusion preventing layer of the first bonding part is formed of amaterial containing, as its main component, a platinum group metal suchas Pt, Rh, Pd, Ir, Ru, or Os. Since the material containing the platinumgroup metal as its main component is low in wettability with AuSn, themolten AuSn solder is not likely to spread on the surface of thediffusion preventing layer. In addition, the material containing theplatinum group metal as its main component can prevent the AuSn frompassing through the diffusion preventing layer and diffusing to thelower layer of the diffusion preventing layer.

According to one or more embodiments of the present invention, thebonding layer of the first bonding part is formed of a materialcontaining Au as its main component. This is because it is compatiblewith AuSn.

In addition, an Au layer may be provided between the adhesive layer andthe diffusion preventing layer. The diffusion preventing layer is strongin stress and could peel off, but by forming the Au layer between theadhesive layer and the diffusion preventing layer, the diffusionpreventing layer can be reduced in stress and prevented from peelingoff.

In addition, according to a method for bonding the wafers in one or moreembodiments of the present invention, in the step of forming the firstbonding part on the surface of the first wafer, an opening/closingcontact point is formed by stacking layers formed of the same materialsin the same order as those of the first bonding part, on the surface ofthe first wafer. According to one or more embodiments of the presentinvention, each layer of the opening/closing contact point is providedto be the same in thickness and to be the same in height from thesurface of the first wafer as the corresponding layer in the firstbonding part. Accordingly, the opening/closing contact point can beproduced at the same time as the step of forming the first bonding part,so that manufacturing cost can be reduced.

According to a method for bonding the wafers according to one or moreembodiments of the present invention, a face having contact with thesecond wafer in the second bonding part is composed of a conductivelayer formed of a material having high chemical resistance (such as oneor more materials selected from Ti, Tin, W, or platinum group material),a via hole is formed in the second wafer in a position corresponding tothe second bonding part, and a penetration wiring is formed in the viahole to be connected to the conductive layer formed of the materialhaving the high chemical resistance. There is a case where asemiconductor integrated circuit or an actuator is provided on thesecond wafer and connected to the second bonding part of the secondwafer, or the actuator or the semiconductor integrated circuit isprovided on the first wafer and connected to the second bonding partthrough the first bonding part in some cases. In this case, the secondbonding part is connected to a bump provided on an outer face of thesecond wafer through the penetration wiring formed in the via holeprovided in the second wafer in some cases. Even in this case, since theface which is in contact with the second wafer in the second bondingpart is composed of the conductive layer formed of the material havinghigh chemical resistance, the second bonding part is not likely to bedamaged by an etching solution used at the time of opening the via holein the second wafer, or a plating solution used at the time of formingthe penetration wiring.

In addition, by providing the conductive layer formed of one or morematerials selected from Al, Cu, Ni, W, or polysilicon and having arelatively large thickness in the second bonding part, a larger spacecan be formed between the first wafer and the second wafer.

In addition, by forming an Au layer on a surface of the second bondingpart, and covering an outer periphery face and an outer periphery edgeof the surface of the second bonding part with an insulating coatingfilm, an exposed area of the surface of the second bonding part isreduced, so that the AuSn is not likely to spread on the surface of thesecond bonding part when the second bonding part is connected by theAuSn eutectic bonding.

According to a structure of a first bonding part in one or moreembodiments of the present invention, a diffusion preventing layerformed of a material having low wettability with AuSn is stacked above awafer, a bonding layer is formed on a surface of the diffusionpreventing layer in such a manner that the bonding layer stays back ofan edge of the diffusion preventing layer, and a functional layer easilydeteriorating due to diffusion of the AuSn is formed between the waferand the diffusion preventing layer. According to the structure of thebonding part, since the bonding layer provided on the surface of thediffusion preventing layer formed of the material having the lowwettability with the AuSn is formed so as to stay back of the edge ofthe diffusion preventing layer, a molten AuSn solder is not likely tospread on the surface of the diffusion preventing layer when the firstbonding part is connected by AuSn eutectic bonding, and the AuSn solderis not likely to flow into the functional layer which easilydeteriorates due to the diffusion of the AuSn. Therefore, the functionallayer hardly deteriorates because the AuSn does not diffuse into thefunctional layer.

According to one or more embodiments of the present invention, thediffusion preventing layer of the first bonding part is formed of amaterial containing, as its main component, a platinum group metal suchas Pt, Rh, Pd, Ir, Ru, or Os. Since the material containing the platinumgroup metal as its main component is low in wettability with the AuSn,the molten AuSn solder is not likely to spread on the surface of thediffusion preventing layer. In addition, the material containing theplatinum group metal as its main component can prevent the AuSn passingthrough the diffusion preventing layer and diffusing to the lower layerof the diffusion preventing layer.

The functional layer of the first bonding part is an adhesive layerformed on a surface of the first wafer and containing Cr as its maincomponent. Since the Cr is high in adhesiveness with the wafer, thefirst bonding part and the first wafer can be firmly adhered. When theAuSn diffuses into the adhesive layer, the adhesiveness of the adhesivelayer deteriorates, but according to one or more embodiments of thepresent invention, the AuSn is prevented from diffusing into theadhesive layer.

According to one or more embodiments of the present invention, thebonding layer of the first bonding part is formed of a materialcontaining Au as its main component. This is because it is compatiblewith the AuSn.

According to a structure of the bonding part according to one or moreembodiments of the present invention, an opening/closing contact pointis formed by stacking layers composed of the same materials in the sameorder as those of the first bonding part, on the surface of the firstwafer. According to one or more embodiments of the present invention,each layer of the opening/closing contact point has the same thicknessand the same height from the surface of the first wafer as those of thecorresponding layer in the first bonding part. Accordingly, theopening/closing contact point can be produced at the same time as thefirst bonding part in the same step, so that the manufacturing cost canbe low.

A structure of a second bonding part in one or more embodiments of thepresent invention has a first conductive layer formed of a materialhaving high chemical resistance on a surface of a wafer, a secondconductive layer formed on the first conductive layer and having arelatively large thickness, and a barrier layer formed on the secondconductive layer. According to the structure of the bonding part, thesurface which is in contact with the wafer is composed of the conducivelayer formed of the material having the high chemical resistance, sothat the bonding part is not damaged by the etching solution used at thetime of opening the via hole in the wafer, or the plating solution usedat the time of forming the penetration wiring.

In addition, by forming an electronic component structure body in thepair of wafers bonded by the method for bonding the wafers according toone or more embodiments of the present invention, an electroniccomponent can be produced.

Embodiments formed by combining or modifying disclosed embodiments arewithin a scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic cross-sectional views to describe a methodfor bonding wafers according to one or more embodiments of the presentinvention.

FIGS. 2A and 2B are schematic cross-sectional views to describe a methodfor bonding wafers according to a comparison example.

FIG. 3A is an infrared photograph showing a state of an adhesive layerafter the wafers have been bonded in the comparison example. FIG. 3B isan infrared photograph showing a state of an adhesive layer after thewafers have been bonded in one or more embodiments of the presentinvention.

FIGS. 4A and 4B are schematic cross-sectional views to describe steps offorming a penetration wiring in a second wafer after the wafers havebeen bonded to each other.

FIGS. 5A and 5B are schematic cross-sectional views to describe steps offorming the penetration wiring in the second wafer after the wafers havebeen bonded to each other, after the step shown in FIG. 4B.

FIGS. 6A and 6B are schematic cross-sectional views to describe steps offorming the penetration wiring in the second wafer after the wafers havebeen bonded to each other, after the step shown in FIG. 5B.

FIG. 7 is a schematic cross-sectional view of an electrostatic relayproduced by the method for bonding the wafers according to the presentinvention.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. However, the present inventionis not limited to the following embodiments, and can be variouslydesigned without departing from the scope of the present invention. Inembodiments of the invention, numerous specific details are set forth inorder to provide a more thorough understanding of the invention.However, it will be apparent to one of ordinary skill in the art thatthe invention may be practiced without these specific details. In otherinstances, well-known features have not been described in detail toavoid obscuring the invention.

Structure of Bonding Part and Bonding Method

FIGS. 1A and 1B are schematic cross-sectional views to describe a methodfor bonding wafers according to one or more embodiments of the presentinvention. FIG. 1A shows a structure before a first wafer and a secondwafer are bonded, and FIG. 1B shows a structure after the wafers havebeen bonded.

As shown in FIG. 1A, a first bonding part, that is, a bonding part 3 isprovided on a surface of a first wafer, that is, a wafer 1. The wafer 1is a Si wafer, and an insulating layer 2 of SiO₂ or SiN is formed on itssurface. In addition, an adhesive layer 4 is formed on an upper face ofthe insulating layer 2, and an Au layer 6 is formed on an upper face ofthe adhesive layer 4. Furthermore, a diffusion preventing layer 7 and abonding layer 8 are sequentially formed on the Au layer 6. Thus, thebonding part 3 is composed of the adhesive layer 4, the Au layer 6, thediffusion preventing layer 7, and the bonding layer 8. The bonding part3 may be in a shape of a block, or may be elongated into a shape of aband.

The adhesive layer 4 is a Cr thin film formed on the surface of theinsulating layer 2 by sputtering. The Cr adhesive layer 4 is provided toensure adhesiveness between the bonding part 3 and the insulating layer2 to prevent the bonding part 3 from peeling off.

The diffusion preventing layer 7 formed on the Au layer 6 is formed of amaterial having low wettability with AuSn, that is, a materialcontaining, as its main component, a platinum group metal such as Pt,Rh, Pd, Ir, Ru, or Os. The diffusion preventing layer 7 serves as abarrier layer to prevent an AuSn solder from diffusing into the Au layer6.

The diffusion preventing layer 7 easily peels off due to its strongstress, so that when it is directly formed on the upper face of theadhesive layer 4, the diffusion preventing layer 7 could peel off.Therefore, in the bonding part 3, by sandwiching the Au layer 6 which issoft and relatively thick, between the adhesive layer 4 and thediffusion preventing layer 7, the stress of the diffusion preventinglayer 7 is relieved and the diffusion preventing layer 7 is adheredthereto. In addition, by providing the Au layer 6 having small electricresistance, the bonding part 3 can be used as a wiring.

The bonding layer 8 is formed as an Au-plated film to provide eutecticbonding with the AuSn solder. In addition, the bonding layer 8 is formedso as to stay back of an edge of the diffusion preventing layer 7. Thatis, in the case where the bonding part 3 is in the shape of the block,an entire periphery of the bonding layer 8 stays back of an outerperiphery side of the diffusion preventing layer 7. In addition, in thecase where the bonding part 3 is in the shape of the band, both edges ofthe bonding layer 8 in a width direction stay back of both edges of thediffusion preventing layer 7.

As shown in FIG. 1A, a second bonding part, that is, a bonding part 13is provided on a lower face of a second wafer, that is, a wafer 11. Thewafer 11 is a Si wafer, and an insulating layer 12 of SiO₂ or SiN isformed on the lower face. In addition, a conductive layer which is highin chemical resistance (especially, remain unaltered by an acid), thatis, a first conductive layer 14 is formed on a lower face of theinsulating layer 2, a conductive layer formed of a material which caneasily become thick, that is, a second conductive layer 15 is formed ona lower face of the first conducive layer 14, and a barrier layer 16 isformed on a lower face of the second conductive layer 15. Thus, thebonding part 13 is composed of the first conductive layer 14, the secondconductive layer 15, and the barrier layer 16. The bonding part 13 alsomay be in a shape of a block or may be elongated into a shape of a band.

The first conductive layer 14 is formed of a conductive material havinghigh chemical resistance. The first conductive layer 14 is formed of Ti,TiN, W, or a platinum group material. According to one or moreembodiments of the present invention, Ti which has high resistance tocopper sulfate serving as a plating solution and high adhesiveness withthe insulating layer 12 is used for the first conductive layer 14, andits thickness is 500 to 1000 Å. The second conductive layer 15 is formedof a material which can be relatively large in thickness and convenientin forming a control circuit or a wiring. The second conductive layer 15only has to be able to be formed thickly by a film forming method suchas plating, sputtering, or CVD. For example, it is formed of Al, Cu, Ni,W, or polysilicon to be 2 μm in thickness. According to one or moreembodiments of the present invention, the Al is used because it isinexpensive, versatile, and high in productivity. Meanwhile, in a casewhere a high-frequency signal is transmitted to the bonding part 13,according to one or more embodiments of the present invention, Ni, whichis a magnetic material is not used. According to one or more embodimentsof the present invention, when the Al is used, 5% by weight of Cu isadded to prevent Al spike. The barrier layer 16 prevents Au fromdiffusing to the second conducive layer 15, prevents the material (Al)of the second conductive layer 15 from diffusing to Au of a Ti/Au layer18, and prevents the wiring material and the bonding material from beingmixed. The barrier layer 16 is formed of TiN having high barrierproperties to be 400 Å in thickness.

An outer periphery face and an outer periphery part of a lower face ofthe bonding part 13 is covered with an insulating coating film 17composed of SiO₂ or SiN, and a part of the lower face of the bondingpart 13 (barrier layer 16) is not covered with the insulating coatingfilm 17. Here, an opening width of the insulating coating film 17 issmaller than a width of the barrier layer 16. In addition, the Ti/Aulayer 18 composed of a lower layer Ti and an upper layer Au is formed onthe lower face of the bonding part 13 so as to be larger in area than anopening of the insulating coating film 17. A width of the Ti/Au layer 18is larger than the opening width of the insulating coating film 17 butsmaller than the width of the barrier layer 16. An AuSn solder layer 19is formed on a lower face of the Ti/Au layer 18 in such a manner that anSn layer 20 and an Au layer 21 are alternately stacked. In addition, theAuSn solder layer 19 may be formed of an AuSn alloy.

As shown in FIG. 1A, the wafer 1 having the bonding part 3 on its upperface and the wafer 11 having the bonding part 13 on its lower face areopposed to each other, and the AuSn solder layer 19 provided on thelower face of the bonding part 13 is stacked on an upper face of thebonding part 3. In this state, the wafer 11 is pressed against the wafer1 at an appropriate pressure under the condition that the AuSn solderlayer 19 is melted. As a result, as shown in FIG. 1B, the Au layer ofthe Ti/Au layer 18 and the bonding layer 8 (Au layer) are meltedtogether with the AuSn solder layer 19 and become an AuSn solder 22, andthe bonding part 13 and the bonding part 3 are bonded by the eutecticbonding.

Since the bonding layer 8 stays back of the edge of the diffusionpreventing layer 7, and the diffusion preventing layer 7 containing theplatinum group metal as its main component is low in wettability withAuSn, the molten AuSn solder 22 does not spread to the edge of thediffusion preventing layer 7. Therefore, the AuSn solder 22 does notflow to a side face of the bonding part 3 and not reach the adhesivelayer 4, so that the AuSn does not diffuse into the adhesive layer 4.Thus, the adhesiveness of the adhesive layer 4 is not damaged becausethe AuSn does not diffuse into the adhesive layer 4.

In addition, since the outer periphery part of the lower face of thebarrier layer 16 is covered with the insulating coating film 17 and theexposed area of the barrier layer 16 is reduced, the AuSn solder 22 isalso not likely to spread in the bonding part 13.

In addition, when the film thickness of the second conductive layer 15is large, the space between the wafers 1 and 11 can be large.

Comparison Between One or More Embodiments of the Present Invention andComparison Example

Next, a description will be given of a case where the bonding parts 3and 13 are bonded by AuSn eutectic bonding by a method for bondingwafers in a comparison example. FIG. 2A shows a structure before a firstwafer and a second wafer are bonded in the comparison example, and FIG.2B shows a defect after the wafers have been bonded in the comparisonexample.

A difference between one or more embodiments of the present inventionand the comparison example is in that while the width of the bondinglayer 8 is made shorter than that of the diffusion preventing layer 7 inthe bonding part 3, and the bonding layer 8 stays back of the edge ofthe diffusion preventing layer 7 in one or more embodiments of thepresent invention as shown in FIG. 1A, the bonding layer 8 and thediffusion preventing layer 7 are the same in width and the edge of thebonding layer 8 coincides with the edge of the diffusion preventinglayer 7 in the comparison example as shown in FIG. 2A.

According to the comparison example 2, the bonding layer 8 covers thewhole diffusion preventing layer 7, so that when the AuSn solder layer19 is melted, the molten AuSn solder 22 could spread to the end of thediffusion preventing layer 7 and drop along the side face of the bondingpart 3 beyond the end of the diffusion preventing layer 7. Thus, whenthe molten AuSn solder 22 comes into contact with the adhesive layer 4,the AuSn diffuses into the adhesive layer 4, which hinders theadhesiveness between the adhesive layer 4 and the wafer 1. As a result,according to the comparison example, there is a reduction in yield of abonded product of the wafers.

On the other hand, according to one or more embodiments of the presentinvention, since the diffusion preventing layer 7 which is not coveredwith the bonding layer 8 is low in wettability with AuSn, the AuSnsolder 22 does not spread to the end of diffusion preventing layer 7,and the AuSn is not likely to diffuse into the adhesive layer 4.

FIG. 3A is an infrared photograph showing a state of the adhesive layer4 in the case where the wafers are bonded by AuSn bonding by the methodin the comparison example. FIG. 3B is an infrared photograph showing astate of the adhesive layer 4 in the case where the wafers are bonded bythe AuSn bonding by the method in one or more embodiments of the presentinvention. FIG. 3A and FIG. 3B each shows a boundary between theadhesive layer 4 and the insulating layer 2 taken from a lower faceside.

As shown in FIG. 3A in the comparison example, the AuSn diffuses intothe adhesive layer 4, so that the adhesive layer 4 considerablydeteriorates. Especially, a part shown by K severely deteriorates due tothe diffusion. Meanwhile, as shown in FIG. 3B in one or more embodimentsof the present invention, the adhesive layer 4 keeps a clean and smoothstate.

Case where Via Hole and Penetration Wiring are Provided

The bonding structure shown in FIG. 1B is good enough for a part inwhich the AuSn eutectic bonding is provided only between the bondingparts of the wafers (such as a bonding part for sealing outer peripheryparts of the wafers in an electrostatic relay which will be describedlater). However, there is a case where a semiconductor integratedcircuit is formed on the lower face of the wafer 11 (second wafer) andthe semiconductor integrated circuit is connected to the bonding part13. In this case, it is necessary to form a via hole in the wafer 11,and connect the bonding part 13 to a bump provided on an upper face ofthe wafer 11 through a penetration wiring provided in the via hole. Inaddition, there is a case where an actuator is provided on the wafer 1(first wafer), and an electrode of the actuator is electricallyconnected to the bonding part 13 through the conductive bonding part 3.In this case also, it is necessary to form the via hole in the wafer 11,and connect the bonding part 13 to the bump provided on the upper faceof the wafer 11 through the penetration wiring provided in the via hole.

Thus, in the case where the via hole and the penetration wiring have tobe provided in the wafer, after the wafers 1 and 11 have been bonded,the wafer 11 is to be processed as shown in FIGS. 4A to 6B.

FIGS. 4A to 6B show steps of providing a via hole 32 and a penetrationwiring 33 in the wafer 11 after the wafers 1 and 11 have been bondedaccording to one or more embodiments of the present invention.Hereinafter, these steps will be described with reference to FIGS. 4A to6B.

First, as shown in FIG. 4A, an insulating layer 31 is formed of SiO₂ orSiN on the upper face of the wafer 11, and the via hole 32 is formed inthe wafer 11 just above the bonding part 13, so that the insulatinglayer 12 is exposed to a bottom face of the via hole 32.

Then, as shown in FIG. 4B, by evaporating SiO₂ or SiN on a whole surfaceof the wafer 11, the insulating layer 31 is formed on an inner face ofthe via hole 32, and the insulating layer 31 on the upper surface of thewafer 11 is thickened.

Then, as shown in FIG. 5A, the insulating layer 31 and the insulatinglayer 12 on the bottom face of the via hole 32 are removed by etching,and the first conductive layer 14 is exposed to the bottom face of thevia hole 32.

Then, as shown in FIG. 5B, the penetration wiring 33 is formed byplating a conductive material such as Cu or Al on a surface ranging fromthe bottom face and the inner peripheral face of the via hole 32 to anupper face of the insulating layer 31, and then as shown in FIG. 6A, thepenetration wiring 33 is formed into a predetermined pattern by etching.The first conductive layer 14 is formed of Ti, TiN, or W which is highin chemical resistance, so that when the via hole 32 is formed, and thepenetration wiring 33 is formed by plating, an etching solution and aplating solution are stopped by the first conductive layer 14, and thebonding part 13 is protected from the etching solution and the platingsolution.

Then, as shown in FIG. 6B, the upper face of the wafer 11 is coveredwith a protective film 34 formed of polyimide, the protective film 34 ispartially opened to expose the penetration wiring 33, and a bump 35 isformed on the exposed part of the penetration wiring 33.

Case of Electrostatic Relay

An electrostatic relay 41 (electronic component) will be described as aspecific example of a bonded body of the wafers. FIG. 7 is a schematiccross-sectional view of the electrostatic relay.

The electrostatic relay 41 has been produced on the wafer 1 by MEMStechnique. The electrostatic relay 41 is composed of a fixed contactpart 46 and a movable contact part 45 (each serves as an opening/closingcontact point) arranged so as to be opposed to each other, and anelectrostatic actuator 42 which translates the movable contact part 45by electrostatic force. The electrostatic actuator 42 is composed of afixed part 43 and a movable part 44 produced in one part of the wafer 1.The fixed part 43 has a comb-like teeth structure in plural rows, andformed integrally with the wafer 1. The movable part 44 also has acomb-like teeth structure in plural rows, and is supported in a stateseparated from the wafer 1 so that it can be horizontally moved back andforth. The comb-like teeth structure of the fixed part 43 and thecomb-like teeth structure of the movable part 44 engage with each otherso as not to come in contact. Thus, when a voltage is applied betweenthe comb-like teeth structures, the movable part 44 is pulled toward thefixed part 43 and the movable part 44 is horizontally moved by theelectrostatic force generated between the comb-like teeth structure ofthe fixed part 43 and the comb-like teeth structure of the movable part44.

A bonding part 47 is positioned in each of outer periphery edges of thewafers 1 and 11, and seals a space between the wafers 1 and the wafer11. The bonding part 47 is provided by bonding the bonding part 3 andthe bonding part 13 with the AuSn solder 22 by the eutectic bonding.

A bonding part 48 is a part to electrically connect the electrostaticactuator 42 to the penetration wiring 33 and the bump 35. This bondingpart 48 is also provided by bonding the bonding part 3 and the bondingpart 13 with the AuSn solder 22 by the eutectic bonding.

The movable contact part 45 is provided on an upper face of the movablepart 44 of the electrostatic actuator 42. The fixed contact part 46 isprovided on the upper face of the wafer 1 so as to be opposed to themovable contact part 45. The movable contact part 45 and the fixedcontact part 46 are not the bonding parts, but they are formed at thesame time as the bonding part 3, and have the same layer structure asthe bonding part 3. That is, each of the movable contact part 45 and thefixed contact part 46 is composed by sequentially stacking the adhesivelayer 4, the Au layer 6, the diffusion preventing layer 7, and thebonding layer 8, and each layer has the same thickness and height fromthe surface of the wafer 1 as the corresponding layer in the bondingpart 3. Thus, a layer containing the platinum group metal as its maincomponent (corresponding to the diffusion preventing layer 7) serves asa fixed contact point 45 a and a movable contact point 46 a. Inaddition, a layer formed of Au (corresponding to the Au layer 6) is awiring part which runs parallel to the upper face of the wafer 1 and isconnected to the fixed contact point 45 a or the movable contact point46 a.

In addition, by increasing the thickness of the second conductive layer15 formed of Al in the bonding part 13, a height of the space betweenthe wafers 1 and 11 can increase, so that the movable contact part 45can be prevented from interfering with the wafer 11 when the movablecontact part 45 horizontally moves.

In addition, since the width of the bonding layer 8 is smaller than thewidth of the diffusion preventing layer 7, and the AuSn solder is notlikely to flow to the lower part of the bonding part 3, the movable part44 is prevented from getting stuck because the AuSn solder does not flowto the movable section 44.

In addition, a process circuit to process a detection signal outputtedfrom the electrostatic relay 41 is provided on the lower face of thewafer 11, but it is not shown in FIG. 7.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

DESCRIPTION OF SYMBOLS

-   1 Wafer (First Wafer)-   3 Bonding Part (First Bonding Part)-   4 Adhesive Layer-   6 Au Layer-   7 Diffusion Preventing Layer-   8 Bonding Layer-   11 Wafer (Second Wafer)-   13 Bonding Part (Second Bonding Part)-   14 First Conductive Layer-   15 Second Conductive Layer-   16 Barrier Layer-   17 Insulating Coating Film-   18 Ti/Au Layer-   19 AuSn Solder Layer-   20 Sn Layer-   21 Au Layer-   22 AuSn Solder-   31 Insulating Layer-   32 Via Hole-   33 Penetration Wiring-   34 Protective Layer-   35 Bump

What is claimed is:
 1. A method for bonding wafers, comprising: forminga first bonding part on a surface of a first wafer by stacking adiffusion preventing layer formed of a material having low wettabilitywith AuSn above the first wafer and forming a bonding layer on a surfaceof the diffusion preventing layer such that the bonding layer stays backof an edge of the diffusion preventing layer; forming a second bondingpart on a surface of a second wafer; and bonding the first bonding partand the second bonding part by eutectic bonding with an AuSn solderunder a condition that the first wafer and the second wafer are opposedto each other.
 2. The method for bonding the wafers according to claim1, wherein the diffusion preventing layer of the first bonding part isformed of a material containing, as a main component thereof, a platinumgroup metal.
 3. The method for bonding the wafers according to claim 1,wherein the bonding layer of the first bonding part is formed of amaterial containing Au as a main component thereof.
 4. The method forbonding the wafers according to claim 1, further comprising: an adhesivelayer containing Cr as a main component thereof is formed on the surfaceof the first wafer.
 5. The method for bonding the wafers according toclaim 4, further comprising: an Au layer is formed between the adhesivelayer and the diffusion preventing layer.
 6. The method for bonding thewafers according to claim 1, further comprising: an AuSn solder layer ispreviously formed on at least one surface of the first bonding part andthe second bonding part by alternately stacking Au and Sn or with anAuSn alloy, and the first bonding part and the second bonding part arebonded by AuSn eutectic bonding with the molten AuSn solder layer. 7.The method for bonding the wafers according to claim 1, wherein in thestep of forming the first bonding part on the surface of the firstwafer, an opening/closing contact point is formed by stacking layersformed of the same materials in the same order as those of the firstbonding part, on the surface of the first wafer.
 8. The method forbonding the wafers according to claim 7, wherein each layer of theopening/closing contact point has the same thickness and the same heightfrom the surface of the first wafer as those of the corresponding layerin the first bonding part.
 9. The method for bonding the wafersaccording to claim 1, further comprising: a face having contact with thesecond wafer in the second bonding part, comprising a conductive layerformed of a material having high chemical resistance, a via hole formedin the second wafer in a position corresponding to the second bondingpart, and a penetration wiring formed in the via hole to be connected tothe conductive layer formed of the material having the high chemicalresistance.
 10. The method for bonding the wafers according to claim 1,wherein the second bonding part has a conductive layer having arelatively large thickness.
 11. The method for bonding the wafersaccording to claim 1, wherein a surface of the second bonding partcomprises an Au layer, and wherein an outer periphery face and an outerperiphery edge of the surface of the second bonding part are coveredwith an insulating coating film.
 12. The method for bonding the wafersaccording to claim 9, wherein the material having the high chemicalresistance is one or more materials selected from Ti, TiN, W, andplatinum group material.
 13. The method for bonding the wafers accordingto claim 10, wherein the conductive layer having the relatively largethickness is formed of one or more materials selected from Al, Cu, Ni,W, and polysilicon.